"Mass" or "media" finishing are two of several terms that are used to describe techniques for finishing the surfaces of objects, particularly metal workpieces, by physically contacting the objects with a collection of solid particles, with or without the use of additional chemicals to enhance the process.
In its basic methods, mass finishing has its roots in antiquity; e.g., polishing metal articles, such as swords and armor, in rotating barrels of sand or stones.
Current techniques are generally more sophisticated and usually incorporate a motor-driven vibrating container that holds a specialized media along with appropriate chemicals that enhance the finishing capability and action of the media. U.S. Pat. Nos. 3,516,203 and 3,566,552 are exemplary of more recent techniques. Theoretically, the goal of finishing is to reduce or eliminate the amount of hand finishing that must be applied to metal articles, including articles that are to be further plated, for example, with chromium plating such as is commonly used in automobile parts. In particular, the open-style alloy wheels that have become more popular on all varieties of automobiles in recent decades are often formed in a casting technique that leaves a somewhat coarse (or indeed very coarse) surface. The coarse surface then requires appropriate treatment either to give a final appearance, or to prepare the surface for further plating or coating processes. These wheels tend to be difficult to completely finish in conventional media finishing machines, however, and usually require several steps of hand polishing and buffing, both before and after any media finishing before they are suitable either for sale as finished or after plating, painting, or other coating.
One more recent technique for media finishing is often referred to as "drag finishing." In this technique, the workpieces to be finished are immersed in and then pulled through the media, and usually without media vibration as the objects are being pulled through it (hence the term "drag"). In a typical drag conveyor, the workpieces are also rotated on spindles as they are being dragged. U.S. Pat. No. 5,251,409 is exemplary of drag finishing apparatus and techniques.
Although drag finishing is an improvement over certain earlier techniques, it creates a characteristic problem in which the workpieces being polished tend to leave the equivalent of a wake behind them as they are dragged through the media, typically in a revolving circular pattern. Although the presence of the wake poses no practical problem when a single workpiece is being drag finished, it has significant disadvantages when several pieces or groups of pieces are following in each other's wake in a drag conveyor. The skewed pattern of the media in the wake keeps the media from presenting itself to the full face of the object being polished (or vice versa), thus either reducing the quality of the finished part or greatly extending the time required to carry out the finish, or both.
If the mass finishing process fails to remove sufficient material or otherwise fails to properly polish the surface, the workpiece generally must be hand finished with small finishing tools. The hand finishing process tends to be labor intensive, relatively slow, and generally expensive. Additionally, the hand finishing can discharge metals into the ambient surroundings. Accordingly, some jurisdictions establish regulatory limits as to how much hand finishing of metal pieces can be carried out on a periodic timed basis.
As noted above, chrome-plated wheels formed of aluminum alloy are becoming increasingly demanded in the automotive marketplace. Such wheels are typically difficult to finish because of the stylistic openings ("windows") in the wheels which often require much hand finishing. If the wheels are intended to be chrome plated, they tend to take the plating less favorably in the "low current density" area of the wheel. As a result, the chrome plating process can magnify, rather than reduce, the coarse or rough appearance in those areas. Additionally, because the chrome plating does not cover poorly finished areas very well, the prior nickel plating that supports the chrome tends to show up as a yellow-tinted area highlighting the poor plating quality of the wheel. As a result, wheels typically do not carry the level of warranty as do other parts of an automobile, and that original equipment manufacturers (OEM) often prefer to give.
In addition to OEM wheels, there is a relatively large aftermarket, particularly in the United States, for customized alloy wheels.
More specifically, sharp edges tend to be more difficult to plate or coat. Thus to the extent that a mass finishing process fails to moderate such edges, the later finishing steps will remain more difficult.
As another disadvantage, plated and clear coated finishes all tend to exhibit disadvantages at certain edges and corners of three-dimensional objects.
Accordingly, a need exists for a mass finishing technique that can successfully and completely finish all of the custom and difficult-shaped portions of certain object such as automobile wheels and do so in a manner that either successfully supports later plating or coating, or that produces a finished wheel that has little or no need of hand finishing (or of other mechanical finishing such as relatively expensive robotic belting or buffing machines) prior to marketing and use.
The invention meets this object with an apparatus and associated method for finishing the surfaces of workpieces and that is particularly suitable for metal workpieces with complex shapes such as automobile wheels. The apparatus comprises a tub, finishing media in the tub, means for moving the media in the tub in a generally revolving motion in the tub, and means for positioning and rotating a workpiece to be polished in the media about an axis that is oblique to the axis about which the media revolves, and without moving the position of the workpiece with respect to the tub as the workpiece rotates.
In another aspect, the invention is the method of mass finishing objects that comprises positioning an object in a tub of media, moving the media in a generally circular and revolving path that intersects the position of the object, and rotating the object in the media about an axis that is oblique to the axis of revolution of the media without moving the position of the object with respect to the tub.
The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the following detailed description taken in conjunction with the accompanying drawings in which: